A large part of the communications today involves the use of autonomously driven vehicles at a common system of roadways. Traffic surveillance has become very important indeed in order to provide for efficient planning of refurnishing or re-construction of roadways as well as to provide for improved safety. There are today many types of control systems for supervision of different types of traffic streams and for many different purposes.
There are systems based on visual surveillance. However, such systems are very expensive and sensitive to wear or damage. Moreover, such systems also require advanced image interpretation in order to enable automatic surveillance. Another system, often used for traffic counting, is to provide flexible tubes over the roadway and monitoring a pressure change in the tubes when a vehicle passes. One disadvantage with such solutions is e.g. their susceptibility for damage and wear, which means that they are unlikely to be used for permanent solutions. Furthermore, the mounting of the equipment may be dangerous, at least at highly busy roads and requires extensive manual operations.
Magnetic sensors are also used for vehicle detection. One simple solution is to provide a loop of electrically conducting material within the roadway surface coating. When a vehicle passes, the vehicle causes disturbances in the earth magnetic field, which in turn causes an induction in the buried loop. A disadvantage of the magnetic loop approach is that it requires relative extensive impact on the roadway surface coating upon installation. Also, if the loop is provided at a relatively shallow depth, it may be destroyed upon wear of the general roadway surface coating. Also in cases where roadway reconditioning is performed, such loops may indeed damage the reconditioning equipment, and have therefore to be removed beforehand.
There are also some prior art traffic sensor system based on different kinds of magnetometers. In the published US patent applications 2005/0190077 and 2006/0132298, methods and apparatuses for vehicular sensors are disclosed. A magnetic sensor using the magnetic resistive effect is provided close to a roadway for sensing a change in magnetic flux when a vehicle passes. The magnetic sensor is comprised in a vehicular sensor node provided at the top of the pavement. A magnetic sensor state is recorded upon the passage of a vehicle and the results are approximated and encoded to be sent to a means for wirelessly receiving. A disadvantage with such a system is that the vehicular sensor node typically is provided on top of the pavement at the side of the roadway, which provides for a relatively weak change in magnetic field. Furthermore, traffic in parallel traffic lanes is difficult to distinguish. Moreover, the vehicular sensor nodes may be susceptible for damage.
In the U.S. Pat. No. 5,877,705, a method and apparatus for analyzing traffic is disclosed. A magnetic sensor is provided buried beneath the road surface and is connected to a data collection computer either via physical conductors or via an RF link to an intermediate roadside receiver. To have physical connections causes problems upon installation, since significant impact has to be provided on the road surface coating causing heavy disturbances in the traffic. This is overcome by using the radio interface. However, in order to reduce the power consumption to allow for battery powering of the sensor, the roadside receiver has to be provided within e.g. 30 meters. This results in that many roadside receivers have to be provided, which increases the cost and the likelihood for damages.
Also the U.S. Pat. No. 5,880,682 discloses a traffic control system based on magnetic sensors buried below the roadway surface asphalt. The sensor is battery powered and communicates with a receiver positioned at the side of the road. This system is intended to be used together with e.g. a traffic control signalling light, where the need for the roadside receiver is not too cumbersome. However, for temporary solutions or for the provision of a multitude of traffic counting locations, the need for the roadside receiver becomes expensive and requires typically the provision of electrical power to the roadside receiver.
In the published patent application US 2002/0177942, a wireless roadway monitoring system is disclosed. A wireless, in-road traffic sensor system uses sensors for measuring e.g. speed of passing vehicles that are small, low-cost, and rugged. The sensor includes a wireless transmitter and may be configured for installation beneath a roadway surface.
A general problem with prior art traffic surveillance detectors is that they are not very well suited for flexible and/or intermittent usage.